Category Archives: Antimicrobial Resistance

“Do we perform too much antimicrobial susceptibility testing?”

As lab workers, we like to be helpful. In general, we want to provide as good a service as possible. But sometimes I think we try a little too hard…

One of our key areas of work is antimicrobial susceptibility testing. This is our bread and butter of course. This is one thing that we can do but no one else can, and we like to show off our skills! But there are many circumstances where performing antimicrobial susceptibility testing adds little value for the patient and thus unnecessarily uses up valuable laboratory resources.

Polymicrobial cultures The clinical value of antimicrobial susceptibility testing is inversely proportional to the number of different organism types present in the sample. This includes sterile site samples. Many times in my career I have been asked to do susceptibilities on samples which have grown several different organisms. I almost always push back on this. It should very much be the exception as opposed to the norm.

Eye and Ear Swabs Conjunctivitis and otitis externa are primarily managed by topical preparations, which can even be antiseptics as opposed to antibiotics. In-vitro susceptibility testing correlates poorly with response to topical antibiotics. Antimicrobial susceptibility testing on ear and eye swabs should only happen in a small minority of cases.

Enterobacteriaceae, enterococci & pseudomonas in superficial wound swabs These organisms cause infection in only a very small proportion of samples that they are actually found in. Susceptibilities should only be performed when there is compelling evidence from the clinical details that they are causing problems. 

Enterococci in urines In contrast to wounds, enterococci commonly cause urinary tract infections (they can also represent contamination). However, because amoxycillin achieves concentrations in urine which exceed the MICs of most Enterococcus faecalis and Enterococcus faecium isolates (check out this reference), susceptibility testing is essentially futile, unless the clinical details suggest the patient has a penicillin allergy. A simple comment to this effect will suffice.

Beta-haemolytic streptococci Because beta-haemolytic streptococci are inherently susceptible to beta-lactams, susceptibility testing for these antibiotics is somewhat academic in the majority of simple wound/soft tissue infections.  I would do if the clinical details suggested penicillin allergy.

Anaerobes Anaerobes rarely require formal susceptibility testing. Bacteroides fragilis has predictable response to beta-lactam/beta-lactamase inhibitor combinations. and is often part of a polymicrobial infection anyway (see polymicrobial cultures). In our lab anaerobic susceptibility testing is most often performed for C. acnes causing joint infections, where we test penicillin (almost always susceptible, maybe we don’t need to test…) and clindamycin (very occasionally resistant).

Coagulase negative staphylococci from blood cultures Again these should only be performed when it is clear that the coagulase negative staph is the suspected pathogen (prosthetic material, premature neonates, etc.) which will only be the small majority of the total number of isolates.

Pseudomonas in sputa Once a patient with COPD becomes colonised with Pseudomonas aeruginosa in their sputum, it is generally there to stay. Pseudomonas susceptibility testing should only be done when it is clear from the clinical details that it is causing a problem, i.e. the patient is failing standard management. We also need to review susceptibility testing protocols on pseudomonas isolates from patients with bronchiectasis and cystic fibrosis. There is now increasing evidence that annual susceptibility testing on Pseudomonas isolates from Cystic Fibrosis patients is more than sufficient.

Candida from vaginal swabs It’s not just bacteria! Recurrent vaginal candidiasis is a common problem, and we are often asked to perform antifungal susceptibilities on such isolates. In my opinion it is hardly ever justified. Nystatin based topical therapy often works in these patients. Candida albicans isolates are usually susceptible to generous dosing of azoles. It is only Nakaseomyces glabrata (formerly known as Candida glabrata), where I occasionally acquiesce and perform susceptibility testing…

Of course, we can perform antimicrobial susceptibility testing but not report the results, having them stored just in case. But my view is that we should minimise this approach as it is generally wasteful. We should perform antimicrobial susceptibility testing when we are confident that we are going to report the results of at least some of the antibiotics from a testing panel.

At my lab we have progressed a lot in this area over the past decade and now perform minimal amounts of antimicrobial susceptibility testing in all of the areas above. What about your own lab? Is there room for improvement, and can you think of other areas where too much antimicrobial susceptibility testing is performed, that I have not thought of?


“Use and Abuse”

You can produce sophisticated and comprehensive antimicrobial resistance surveillance data.

You can adhere to the best infection control policies in the country.

You can have a “search and destroy” policy for multi-resistant organisms.

and you can even develop and bring out a new antibiotic every couple of years….

But unless you control antibiotic consumption (usage), you will always be fighting an uphill battle.

In order to control antibiotic consumption you need to know how  many antibiotics are being used in the first place.

One of the problems is that antimicrobial resistance surveillance data is produced by microbiologists. Antibiotic consumption data is produced by pharmacists. Antibiotic consumption data even in this day and age can still be difficult to get hold of. Sometimes I wonder if the companies selling the antibiotics to the hospitals have a much better handle on consumption data than the microbiologists do!

Microbiologists and pharmacists need to talk to each other more. It is such a key relationship in the antimicrobial stewardship world.

Antibiotic usage needs to be surveyed and controlled not only at an individual level, but at a national level. Communities and hospitals, humans and animals. It all adds up… Too often I have sat in conferences and seen pretty graphs of antimicrobial resistance data, without complementary antibiotic consumption data to put the resistance data into context. I find it all a bit frustrating…

If reducing antibiotic usage was easy it would already have happened. It’s not easy , and there are good reasons for this. (See this article). This is where objective data is key to monitoring and measuring change. Feedback to the “prescriber” is critical.

Every antimicrobial stewardship committee in the world needs to be aware of their consumption data. Otherwise they are simply not doing their job. Surveillance of antibiotic consumption does not seem to get the same profile as resistance data. This is a shame. I would actually argue that it is the more important of the two….


“Back pocket antibiotic prescriptions: Good or Bad?”

Whenever my children get taken to the GP for a suspected chest/ear/throat/sinus infection, more often than not we are given a “back pocket”/delayed antibiotic prescription to be used only if the child does not improve over the next couple of days.

Is this a good policy?

My analytical way of thinking deduces the following assertions from the decision to prescribe a “back pocket” antibiotic prescription.

  • The doctor has decided that based on the current clinical presentation, the patient doe not require an antibiotic immediately.
  • The patient/guardian now has the primary responsibility as to whether the antibiotic is actually given or not. Is this really a good thing?

Studies have shown that back pocket prescriptions are acted upon about half the time, maybe a bit less than that. Whether or not a back pocket policy reduces antibiotic prescribing depends entirely on what you are comparing it against. If you say that all these patients would otherwise have received an antibiotic straightaway, then of course you can demonstrate a reduction! (Lies, damned lies and statistics…)

So what seems like a good idea may in reality may not be so perfect after all.

Given that only a small minority of such infections ever really need an antibiotic I prefer an educational approach (check out this leaflet), along with advice to come back if not settling in a couple of days’ time for clinical reassessment.

I remain to be convinced that back pocket prescriptions are a truly effective means of antimicrobial stewardship. To me it is more a way of the GP showing that they are doing something for the patient, a bit of a halfway house as such. It may even give out the completely wrong message to the patient. i.e. “I am not interested in seeing you again.”

If we are really serious about antimicrobial stewardship, then I think we need to review the back pocket policy. I think antibiotic prescribing for minor infections needs much tighter regulation. I also think including the clinical indication for the antibiotic on the prescription should be mandatory.

So the antibiotic prescription in my back pocket hardly ever comes out, and my gut feeling is that it shouldn’t even be there in the first place…


Here is an article with a bit more detail on this debate.